1. Technical Field
This disclosure relates to stack capacitors for semiconductor devices and more particularly, to a high conductivity plug for stack capacitors.
2. Description of the Related Art
Semiconductor memory cells include capacitors accessed by transistors to store data. Data is stored by as a high or low bit depending on the state of the capacitor. The capacitor's charge or lack of charge indicates a high or low when accessed to read data, and the capacitor is charged or discharged to write data thereto.
Stacked capacitors are among the types of capacitors used in semiconductor memories. Stacked capacitors are typically located on top of the transistor used to access a storage node of the capacitor as opposed to trench capacitors which are buried in the substrate of the device. As with many electrical devices, high conductivity is beneficial for performance characteristics of stacked capacitors.
In semiconductor memories, such as dynamic random access memories (DRAM), high dielectric constant capacitor formation processes include deposition of highly dielectric materials. In one type of high dielectric constant capacitors, a layer of high dielectric constant materials, such as barium strontium titanium oxide (BSTO), is deposited in an oxidized atmosphere.
Referring to FIGS. 1A and 1B, a structure 2 with stacked capacitors is shown. Stacked capacitors 3 include two electrodes a top electrode or storage node 4, usually platinum (Pt) and an electrode 12 separated by a dielectric layer 18. An access transistor 5 includes a gate 6 which when activated electrically couples a bitline 7 through a bitline contact 8 to a plug 14. Plug 14 connects to electrode 12 through a diffusion barrier 16 which stores charge in electrode 12.
A partial view of a conventional stacked capacitor 10 is shown in FIG 1B. Stacked capacitor 10 includes electrode 12, preferably formed of platinum (Pt). Electrode 12 is separated from plug 14 by diffusion barrier 16. Plug 14 is preferably polycrystalline silicon (polysilicon or poly). During processing, dielectric layer 18 is deposited on electrode 12. Dielectric layer 18 is typically a material with a high dielectric constant, for example BSTO. During the deposition of dielectric layer 18, oxide layers 20 and 21 form which are detrimental to the performance of the stacked capacitor. Diffusion barrier 16 is employed to prevent the formation of oxide layer 21.
Oxide layers 20 and 21 form if:
(a) silicon diffuses through diffusion barrier 16 and reacts with oxygen to form oxide 20 between diffusion barrier 16 and electrode 12; PA1 (b) diffusion barrier 16 materials simply react with oxygen; and PA1 (c) oxygen diffuses through diffusion barrier 16 and reacts with plug 14 to form oxide layer 21 between diffusion barrier 16 and plug 14.
Oxide layers 20 and 21 reduce the capacitance of stacked capacitor 10. Therefore, a need exists for improving capacitance of stacked capacitors by eliminating oxide layers adjacent to a barrier layer formed as a result of processing and diffusion. A further need exists for a method of increasing conductivity of a plug used in stacked capacitors.